Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass., and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels of the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical volumes. The logical volumes may or may not correspond to the actual disk drives.
In some instances, it may be desirable to copy data from one storage device to another. For example, if a host writes data to a first storage device, it may be desirable to copy that data to a second storage device provided in a different location so that if a disaster occurs that renders the first storage device inoperable, the host (or another host) may resume operation using the data of the second storage device. Such a capability is provided, for example, by a Remote Data Facility (RDF) product provided by EMC Corporation of Hopkinton, Mass., e.g., Symmetrix Remote Data Facility (SRDF). With RDF, a first storage device, denoted the “primary storage device” (or “R1”) is coupled to the host. One or more other storage devices, called “secondary storage devices” (or “R2”) receive copies of the data that is written to the primary storage device by the host. The host interacts directly with the primary storage device, but any data changes made to the primary storage device are automatically provided to the one or more secondary storage devices using RDF. The primary and secondary storage devices may be connected by a data link, such as an ESCON link, a Fibre Channel link, and/or a Gigabit Ethernet link. The RDF functionality may be facilitated with an RDF adapter (RA) provided at each of the storage devices.
There may be a number of different types of RDF transmission. Synchronous RDF mode allows synchronous data transfer where, after an initial data write from a host to a primary storage device, the data is transferred from the primary storage device to a secondary storage device using RDF. Receipt of the data is acknowledged by the secondary storage device to the primary storage device which then provides a write acknowledge back to the host for the initial data write. Thus, in synchronous RDF mode, the host does not receive a write acknowledge from the primary storage device until the RDF transfer to the secondary storage device has been completed and acknowledged by the secondary storage device. One product that allows for synchronous data transfers is produced by EMC Corporation of Hopkinton, Mass. and known as SRDF/S, which is further discussed elsewhere herein.
A drawback to synchronous RDF transmission may be that the latency of each of the write operations is increased by waiting for the acknowledgement of the RDF transfer. This problem is exacerbated when there is a long distance between the primary storage device and the secondary storage device; because of transmission delays, the time delay required for making the RDF transfer and then waiting for an acknowledgement back after the transfer is complete may be undesirable.
Another possibility for an RDF system is to have the host write data to the primary storage device and have the primary storage device copy data asynchronously to the secondary storage device in the background. The background copy involves cycling through each of the tracks of the primary storage device sequentially and, when it is determined that a particular block has been modified since the last time that block was copied, the block is transferred from the primary storage device to the secondary storage device. Although this mechanism may attenuate latency problems associated with synchronous and semi-synchronous data transfer modes, a difficulty still exists because there cannot be a guarantee of data consistency between the primary and secondary storage devices. If there are problems, such as a failure of the primary system, the secondary system may have stored therein out-of-order changes that make the data unusable.
One solution to the above-noted problems is a product by EMC Corporation known as SRDF/A in which data sets are transferred to the secondary array at defined intervals. Using SRDF/A, data to be copied from one storage array to another in chunks that are assigned sequence numbers based on when the data was written by the host. The data chunks are then separately transmitted according to the sequence number. Although the remote copy of the data may not be as current as the primary copy, SRDF/A allows replication of data over considerable distances in a manner that avoids the latency issues of synchronous transfers and has reduced bandwidth requirements. Moreover, data ordering problems associated with other types of remote data synchronous transfers are avoided. For further discussion of SRDF/A systems and techniques, see U.S. Pat. Nos. 7,000,086 to Meiri, et al., entitled “Virtual Ordered Writes,” and 7,054,883 to Meiri, et al. entitled “Virtual Ordered Writes for Multiple Storage Devices,” which are both incorporated herein by reference.
Current data replication/mirroring systems may require active/passive operational behavior, meaning that reads and writes from a host occur on one primary storage volume (active), e.g., R1, while host writes may be disabled and reads may be infrequent at the secondary storage volume (passive), e.g., R2. The active R1 controls writes to the passive R2 in a process that may result in issues of currentness/correctness of the data at R2, as noted above. Further, an explicit failover operation is required to activate R2 in the case of failure of R1. Active/passive behavior may be disadvantageous in some situations, in particular for clustered applications in which applications and individual processes may move between servers needing R1 and R2 to be active at the same time, and in which a level of symmetry between the storage volumes may be required that is not effectively achievable with an active/passive system. Existing active/active solutions may require server mirroring and require server CPU cycles and use of the server's network.
Accordingly, it would be desirable to provide for a system that allows for an enhanced data replication and/or mirroring system and that allows more efficient operations and use of storage devices in the system.